U.S. patent number 9,520,260 [Application Number 14/026,697] was granted by the patent office on 2016-12-13 for photo emitter x-ray source array (pexsa).
This patent grant is currently assigned to The Board of Trustees of the Leland Stanford Junior University. The grantee listed for this patent is The Board of Trustees of the Leland Stanford Junior University. Invention is credited to Yao-Te Cheng, Lambertus Hesselink, Juan R. Maldonado, R. Fabian W. Pease, Piero Pianetta, Jason Ryan.
United States Patent |
9,520,260 |
Hesselink , et al. |
December 13, 2016 |
Photo emitter X-ray source array (PeXSA)
Abstract
A photo-emitter x-ray source is provided that includes a
photocathode electron source, a laser light source, where the laser
light source illuminates the photocathode electron source to emit
electrons, and an X-ray target, where the emitted electrons are
focused on the X-ray target, where the X-ray target emits X-rays.
The photocathode electron source can include alkali halides (such
as CsBr and CsI), semiconductors (such as GaAs, InP), and theses
materials modified with rare Earth element (such as Eu) doping,
electron beam bombardment, and X-ray irradiation, and has a form
factor that includes planar, patterned, or optically patterned. The
X-ray target includes a material such as tungsten, copper, rhodium
or molybdenum. The laser light source is pulsed or configured by
light modulators including acousto-optics, mode-locking,
micro-mirror array, and liquid crystals, the photocathode electron
source includes a nano-aperture or nano-particle arrays, where the
nano-aperture is a C-aperture or a circular aperture.
Inventors: |
Hesselink; Lambertus (Atherton,
CA), Pease; R. Fabian W. (Stanford, CA), Pianetta;
Piero (Palo Alto, CA), Maldonado; Juan R. (Menlo Park,
CA), Cheng; Yao-Te (New Taipei, TW), Ryan;
Jason (Stanford, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
The Board of Trustees of the Leland Stanford Junior
University |
Palo Alto |
CA |
US |
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Assignee: |
The Board of Trustees of the Leland
Stanford Junior University (Palo Alto, CA)
|
Family
ID: |
50274453 |
Appl.
No.: |
14/026,697 |
Filed: |
September 13, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20140079188 A1 |
Mar 20, 2014 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61701031 |
Sep 14, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J
35/065 (20130101); H01J 35/147 (20190501); H01J
35/02 (20130101); H01J 35/066 (20190501); H01J
35/116 (20190501) |
Current International
Class: |
H01J
35/06 (20060101); H01J 35/14 (20060101); H01J
35/02 (20060101) |
Field of
Search: |
;378/119,121,122,136-138 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Javier Alda, "Laser and Gaussian Beam Propagation and
Transformation," Encyclopedia of Optical Engineering, 2003, pp.
999-1013. cited by examiner.
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Primary Examiner: Ho; Allen C.
Attorney, Agent or Firm: Lucent Patent Firm
Government Interests
STATEMENT OF GOVERNMENT SPONSORED SUPPORT
This invention was made with Government support under grant (or
contract) no. HSGQDC-12-C-00002 awarded by the Department of
Homeland Security. The Government has certain rights in this
invention.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority from U.S. Provisional Patent
Application 61/701,031 filed Sep. 14, 2012, which is incorporated
herein by reference.
Claims
What is claimed:
1. A photo-emitter x-ray source, comprising: a. a photocathode
electron source; b. a laser light source; c. a beam forming device
comprising a spatial light modulator; d. electron optics; and e. an
X-ray target, wherein said laser light source outputs a beam
directed to said spatial light modulator, wherein said spatial
light modulator forms said beam into an optical spatially patterned
beam, wherein said optical spatially patterned beam illuminates
said photocathode electron source, wherein said photocathode
electron source emits electrons having an electron pattern
according to said spatial light modulator, wherein said electron
optics comprises an electric field, a magnetic field, or said
electric field and said magnetic field disposed to image said
electron pattern onto said X-ray target, wherein said X-ray target
emits a pattern of X-rays, wherein said pattern of X-rays comprise
a patterned partially-coherent X-ray beam.
2. The photo-emitter x-ray source of claim 1, wherein said
photocathode electron source comprises a material selected from the
group consisting of alkali halides, GaAs, InP, rare Earth element
doped alkali halides, rare Earth element doped GaAs, rare Earth
element doped InP, alkali halides modified by electron beam
bombardment, GaAs modified by electron beam bombardment, InP
modified by electron beam bombardment, alkali halides modified by
X-ray irradiation, GaAs modified by X-ray irradiation, and InP
modified by X-ray irradiation.
3. The photo-emitter x-ray source of claim 1, wherein said
photocathode electron source comprises a material capable of
operating at energies below a bandgap of said material through
doped states or color centers created by UV irradiations, X-rays
irradiations, gamma rays irradiations or electron bombardment.
4. The photo-emitter x-ray source of claim 1, wherein said emitted
pattern of X-rays comprise energies below 250 KeV.
5. The photo-emitter x-ray source of claim 1, wherein said electron
optics are configured to focus the electrons emitted from the
photocathode electron source to a spot size in a range between 20
nm to 5 mm.
6. The photo-emitter x-ray source of claim 1, wherein said laser
light source emitting a radiation at a wavelength in a range of 200
nm to 800 nm.
7. The photo-emitter x-ray source of claim 1, wherein said beam
forming device comprises a nano-aperture disposed directly on said
photocathode electron source, wherein said nano-aperture comprises
one of nano-particle arrays, a C-aperture, or a circular
aperture.
8. The photo-emitter X-ray source of claim 1, wherein said X-ray
target comprises a material selected from the group consisting of
tungsten, copper, rhodium and molybdenum.
9. The photo-emitter X-ray source of claim 1, wherein said
photocathode electron source comprises a form factor selected from
the group consisting of planar, patterned, and optically
patterned.
10. The photo-emitter X-ray source of claim 1, wherein said laser
light source is temporally modulated.
Description
FIELD OF THE INVENTION
The present invention relates generally to X-ray imaging, X-ray
spectroscopy, and industrial inspection. More particularly, the
invention relates to a Photo Emitter X-Ray Source Array (PeXSA) for
X-ray imaging.
BACKGROUND OF THE INVENTION
The current approach to differential phase contrast (DPC) is to use
gratings in front of conventional X-ray sources. The gratings are
very difficult to fabricate for energies higher than 50 KeV, and
absorb a considerable amount of X-ray radiation, thereby reducing
the achievable SNR.
What is needed is a device that includes a patterned source so that
the grating is not needed, and that creates a coherent source
enabling interferometric, time resolved measurements such as
shadowgraph or Schlieren measurements of objects. Time resolved
measurements using X-rays are difficult to make with current X-ray
sources, as switching high voltages rapidly, on the order of
picoseconds, is very difficult.
SUMMARY OF THE INVENTION
To address the needs in the art, a photo-emitter x-ray source is
provided that includes a photocathode electron source, a laser
light source, where the laser light source illuminates the
photocathode electron source to emit electrons, and an X-ray
target, where the emitted electrons are focused on the X-ray
target, where the X-ray target emits X-rays.
According to one aspect of the invention, the material of the
photocathode electron source can include alkali halides (such as
CsBr and CsI), semiconductors (such as GaAs, InP), and theses
materials modified with rare Earth element (such as Eu) doping,
electron beam bombardment, and X-ray irradiation.
In a further aspect of the invention, the photocathode electron
source is capable of operating at energies below a bandgap of the
photocathode electron source through doped states or color centers
created by high energy radiations (UV, X-rays, gamma rays) or high
energy particle bombardment (electrons).
According to a further aspect of the invention, the emitted X-rays
has energies below 250 Kev.
In one aspect of the invention, the emitted X-rays are focused to a
spot size in a range between 20 nm to 5 mm.
In yet another aspect of the invention, the photocathode electron
source has a maximum current density of at least 5 A/cm.sup.2,
where the current density is a function of the input optical power
and a cathode pattern area.
According to a further aspect of the invention, the laser light
source has a wavelength in a range of 200 nm to 800 nm.
In one aspect of the invention, a beam from the laser light source
is pulsed or steered according to light modulators that can include
acousto-optics, mode-locking, micro-mirror array, and liquid
crystals.
In a further aspect of the invention, the photocathode electron
source includes a nano-aperture or nano-particle arrays, where the
nano-aperture is a C-aperture or a circular aperture.
According to another aspect of the invention, the X-ray target
includes a material such as tungsten, copper, rhodium or
molybdenum.
In another aspect of the invention, the photocathode electron
source has a form factor that includes planar, patterned, of
optically patterned.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic drawing of the Photo Emitter X-Ray Source
Array for X-ray imaging, according to one embodiment of the
invention.
FIG. 2 shows a schematic drawing of a general layout of the Photo
Emitter X-Ray Source Array for X-ray imaging having a focusing
magnetic field, according to one embodiment of the invention.
FIG. 3 show a schematic drawing of an exemplary photocathode,
according to different embodiments of the invention.
FIG. 4 shows a schematic drawing of an exemplary modifiable PeXSA
using a spatial light modulator, according to different embodiments
of the invention.
DETAILED DESCRIPTION
The current invention enables new X-ray imaging modalities by
creating an X-ray source that can be patterned and modulated at
very high rates over time. According to one embodiment, an X-ray
target is illuminated by an electron beam, where the electron beam
is generated by illuminating a target with a laser source. The
laser source can be pulsed to enable very short X-ray pulse trains.
This new source enables new imaging modalities such as 3-D
differential phase contrast imaging, X-ray point sources with a
spatial resolution of less than 20 nm, and X-ray spectroscopic
imaging by combining both temporal and spatial imaging
modalities.
In one embodiment of the invention, a previously activated material
capable of efficient electron emission with a photon energy less
than the band gap is illuminated by a laser source, such as a 257
nm doubled Ar laser source, or any other suitable source having a
wavelength in a range of 200-800 nm. A pre-sensitized medium emits
an electron beam upon illumination by the laser source. A magnetic
field is used to image the electron beam onto an X-ray target, or
it can be used as an optically induced electron beam in its own
right, having a spot size from 20 nm to 5 millimeters, and with
very high electron beam intensities. If the electron beam is
incident on an X-ray target, such as tungsten, copper, rhodium or
molybdenum then an X-ray beam is generated. By patterning the
cathode and imaging the cathode onto the target using a magnetic
lens, a patterned X-ray beam is generated. By using a preferred 1:1
imaging configuration for the magnetic lens, aberrations are
minimized. In a further embodiment, the patterned X-ray source is
used to steer the X-ray beam. The patterned source enables
partially-coherent X-ray beams to be used in interferometric
measurements without the need for an amplitude gratings in front of
a standard incoherent X-ray source, as is used in the prior
art.
Some exemplary applications of the invention include a differential
phase contrast (DPC) imaging, where both the real and imaginary
part of a material's index of refraction is measured. This leads to
better and more contrast rich images of soft tissue objects or
other objects having close to the same X-ray absorption but
different real part of the refractive index, such as fluids and
home made explosives. Variations in soft tissue such as breast
tissue is also better observed with this imaging technique. A
further application includes probing matter with a nano-sized,
short pulse X-ray beam for spectroscopic imaging with unprecedented
spatial resolution, where the pulsed electron beam source, without
the X-ray target, can also be used for ionization mass
spectroscopy, enabling both time and space dependent
measurements.
In a further exemplary application, the photo emitter electron
source itself can be used as a far field probe for nano-metrology
applications, which cannot be done with a near-field optical
probe.
According to the current invention, the combination of short,
modulated pulse trains with resolutions from nanometers to
macro-scales is unprecedented.
The PeXSA source according the current invention overcomes these
problems of using gratings in front of conventional X-ray sources
by patterning the source so that the grating is not needed. Another
advantage of the PeXSA source is the ability to create a coherent
source enabling interferometric, time resolved measurements such as
shadowgraph or Schlieren measurements of objects. Time resolved
measurements using X-rays are difficult to make with existing X-ray
sources, as switching high voltages rapidly, on the order of
picoseconds, is very difficult.
According to further embodiments of the invention, different
non-linear source materials can be used, such as a previously
activated material capable of efficient electron emission with a
photon energy less than the band gap of CsBr, either undoped or
doped with rare earth elements such as Eu, or Er, or others having
atomic weights from 21 to 71. Instead of patterning the cathode the
anode can be patterned as well. A laser emitting radiation at 405
nm, for example, may be used with CsBr doped with Eu or other rare
earth elements, or pretreated with electron beam bombardment or
X-ray irradiation.
According to different embodiments of the invention, the
photocathode material can include any one or any combination of
alkali halides (such as CsBr and CsI), semiconductors (such as
GaAs, InP), and theses materials modified with rare Earth element
(such as Eu) doping, electron beam bombardment, or X-ray
irradiation.
The invention includes new aspects such as a patterned cathode
X-ray source, illuminated by a laser beam, a pulsed X-ray PeXSA
having the ability to produce pulses from sub-picosecond to DC, a
sub-20 nm X-ray source operating from sub-picosecond to DC, and a
patterned source for use in lithography.
The current invention provides differential phase contrast imaging
of baggage for DHS applications and other industrial inspection
applications. DPC imaging for medical applications, X-ray
spectroscopy with nano-sized spatial resolution, potential X-ray
beam steering, coherent X-ray imaging and metrology.
Turning now to the figures, FIG. 1 shows a schematic drawing of a
photo-emitter X-ray source array (PeXSA), according to one
embodiment of the invention. In one aspect, the PeXSA can be used
for inspection (X-ray scanning security applications) and medical
imaging applications. As shown in this exemplary embodiment, the
laser light source is optically pumped by a 405 nm solid state
laser, incident on CsBr or CsI suitably doped with a rare earth
element or modified with electron beam bombardment to allow
operation at energies below the bandgap. In this embodiment, the
electrons emanating from the CsBr or CsI film are focused onto an
X-ray target to produce X-rays operating at energy levels below 250
KeV, having a potential spot size from 20 nm to 5 mm and power
levels maximum cathode current densities of at least 5
A/cm.sup.2.
In one embodiment, an array of integrated sources can be
implemented. Scaling laws governing larger arrays of hundreds of
X-ray sources will be determined.
According to the current invention, the laser light source for the
x-ray system could be either a single photoemitter or an array of
identical photoemitters controlled by independent laser beams with
acousto-optics (AOM), mode-locking, micro-mirror array, liquid
crystals or any other suitable light modulators. The current
invention allows rapid x-ray gating at frequencies 100's of MHz.
The laser wavelength of a preferred embodiment for excitation of
electrons in the photocathode materials is chosen to be around 405
nm to enable use of low cost commercially available, high-powered
(100's of mW) diode lasers. It is understood that the current
invention uses excitation wavelengths in a range of 200 nm to 800
nm. The photo-emitter structure mated to the illuminating laser
depends on the required x-ray source spot size and power.
The source x-ray power is limited by heat dissipation in the
target, which depends on the required resolution (spot size).
According to the current invention, the maximum power that can be
handled by the target is approximately given by the relationship:
0.8 W times the spot size in microns. The x-ray spot size produced
by solid metal targets depends on the target density and the energy
of the bombarding electrons. From the laser light source point of
view, for electron spot sizes less than the exciting light
wavelength, the C aperture approach is advantageous. For spot sizes
greater than the light wavelength, it may be possible to use
circular apertures on a metal layer deposited on a high thermal
conductivity material like diamond.
According to another embodiment of the invention, the photocathode
utilizing CsBr or CsI films takes advantage of the color centers
generated on alkali halide materials by UV radiation (257 nm). A
band of radiation-induced energy states about 3.8 eV below the
vacuum energy. This allows photoemission with 4.8 eV laser
radiation. According to one aspect, the invention includes doping
the CsBr or CsI films during deposition with the proper elements or
modifying the CsBr or CsI films with electron bombardment after
deposition, one embodiment uses rare earth materials doping or
electron beam bombardment, to induce energy states closer to the
vacuum level and allow operation at longer wavelengths (.about.405
nm).
Doping of CsBr or CsI with materials like Europium generates light
emission bands in the visible range suitable for x-ray plate
detector applications.
Several types of x-ray targets can be implemented for small spot
size applications, according to embodiments of the current
invention. One embodiment includes thin metal pads with the desired
spot size dimensions and the required x-ray emission
characteristics incorporated thereto. The pads are deposited on
solid targets made of low atomic number materials like Berylium,
which allow low x-ray emission in the range of interest. In a
further embodiment, the targets can be made movable so new areas
can be utilized as needed. Another embodiment includes the use of
transmission targets of the proper metal and thickness. Metal pads
can also be deposited on thin Beryllium foils or plates with low
x-ray absorption.
As shown in FIG. 1, a thin metal target is deposited on a smooth
substrate with low x-ray absorption. The light from the 405 nm
laser beams is converted to electrons by the high thermal
conductivity transparent substrate, the negative biased metal or
conductive transparent film, the nano C-apertures in metal
substrate, circular apertures in metal substrate, C-apertures or
rectangular and round micro apertures coated on the doped or e-beam
bombarded CsBr or CsI film. The electrons generated at the
photocathode accelerator electrode are accelerated and focused on
the x-ray target with electron optics having electric and magnetic
fields. In this exemplary embodiment, the operating vacuum varies
between 10.sup.-8 to 10.sup.-10 Torr. The thickness of the Be or Al
window depends on the required x-ray spectrum. As shown, an
optional x-ray shield enclosure around the photo-emitter source is
provided to reduce the CsBr x-ray exposure, which may negatively
affect its performance. This can be achieved utilizing a heavy
metal cage with appropriate openings.
According to one example, a uniform B field of 0.11 T and uniform E
field of 3.3 MV/m. Focus is achieved after 1 cyclotron orbit at
z=30 mm (see FIG. 2).
FIG. 3 show a schematic drawing of an exemplary photocathodes,
according to different embodiments of the invention. The material
of the photocathode electron source can include alkali halides
(such as CsBr and CsI), semiconductors (such as GaAs, InP), and
theses materials modified with rare Earth element (such as Eu)
doping, electron beam bombardment, and X-ray irradiation. Further,
the photocathode electron source is capable of operating at
energies below a bandgap of the photocathode electron source
through doped states or color centers created by high energy
radiations (UV, X-rays, gamma rays) or high energy particle
bombardment (electrons).
In yet another aspect of the invention, the photocathode electron
source has a maximum current density of at least 5 A/cm.sup.2,
where the current density is a function of the input optical power
and a cathode pattern area.
In another aspect of the invention, the photocathode electron
source has a form factor that includes planar, patterned, of
optically patterned.
One embodiment, to reduce heat loading on the x-ray target,
involves illuminating the target with an elliptical electron spot
(usually impinging at 6-12 degrees). This can reduce the heat
loading by more than a factor of 5. Another embodiment uses a
conical target with nucleate boiling to reduce the heat load. This
can be accomplished by etching conical indentations on a flat plate
and bombarding the inside of the cones with electrons while flowing
a high velocity turbulent flow over the back of the cones to
enhance heat transfer.
FIG. 4 shows a schematic drawing of an exemplary modifiable PeXSA
using a spatial light modulator, according to one embodiment of the
invention. Here, the patterned X-ray source can be modifiable if
introducing a spatial light modulator to generate the optical
pattern on the photocathode. As the optical pattern (hence the
X-ray source pattern) can be readily programmed and modified with
time by the spatial light modulator, the shape of the X-ray source
can be no longer limited by the case of fixed-pattern photocathode,
as discussed in FIG. 3. This design can be flexibly repurposed for
many applications.
The present invention has now been described in accordance with
several exemplary embodiments, which are intended to be
illustrative in all aspects, rather than restrictive. Thus, the
present invention is capable of many variations in detailed
implementation, which may be derived from the description contained
herein by a person of ordinary skill in the art. All such
variations are considered to be within the scope and spirit of the
present invention as defined by the following claims and their
legal equivalents.
* * * * *